Chemical characteristics of Upper Cretaceous (Turonian) jet of the Gosau Group of Gams/Hieflau (Styria, Austria)

Jet and coaly sediments within the Upper Cretaceous (Gosau) Schoenleiten Fo rmation were collected from two outcrops near Gams/Hieflau (Styria, Austria ). For comparison, additional jet and coal samples from different Gosau loc alities were included in the study. The identification of jet as bituminous driftwood (collotelinite) has been provided by microscopical examination o f organic matter [Kollmann, H.A. and Sachsenhofer, R.F., Mitt. Ref. Geol. u nd Palaont. Landesmuseum Joanneum SH 2 (1998) 223]. Rock-Eval analyses reve aled enhanced HI and lower T,, values of jet compared with the coals and co aly shales. The results reflect the higher contents of bituminous organic m atter compared to vitrain-rich coals of similar maturity. In comparison wit h the coals and coaly shales, high amounts of liberated hydrocarbons (mg HC /g C-org) are in contrast to generally lower yields of soluble organic matt er during extraction with dichloromethane, indicating that portions of the hydrocarbons are liberated from lipids loosely bound to kerogen during pyro lysis. Further differences in n-alkane distribution patterns, odd over even predom inance, pristane/phytane, and pristane/n-C-17 ratios exist between jet, coa ls and coaly shales. The microbial origin of short-chain n-alkanes presumab ly resulted from the degradation of the predominant odd-numbered long-chain n-alkanes within the jet. The occurrence of biological markers (sesqui and diterpenoids) within the sediments (coaly shales), coals and jet samples i ndicates the origin of terrestrial organic matter from conifers. Biomarker composition of jet samples indicates enhanced aromatisation of sesqui and d iterpenoids. These results correspond to enhanced aromatisation of steranes . as reflected by generally higher relative contents of triaromatic steroid s in the jet. Because of comparable maturation (10.5-0.6% R,) of the coals, coaly shales and the jet samples, confirmed by the isomerisation of alpha beta C-31-hopanes and alpha alpha alpha C-29-steranes, the increase in arom atisation in the jet is thought to reflect increased contribution of bacter ia to organic matter degradation. The lack of enrichment of the jet samples in hopanes relative to steranes suggests that enhanced biomarker aromatisa tion is caused by anaerobic rather than by aerobic bacteria. (C) 2001 Elsev ier Science B.V. All rights reserved.